REPORT ON THE CALCAREA. 13 



Ascones, and this is just the circumstance to which Vosmaer does not pay sufficient 

 attention. " And an Olynthus," says Dr. Vosmaer further, " is nothing but a primitive 

 Ascon." I understand the Olynthus otherwise. The Olynthus is a neutral being, and 

 the Ascon one of its modifications, the Sycon another. An Olynthus may increase 

 longitudinally only, without lateral growth, and in that case it will give origin to an 

 Ascon, the most marked peculiarity of which, in comparison with all other sponges, 

 consists in the slight development of its mesoderm, in other words, in verv marked 

 thinness of its walls. An Olynthus may also grow in all directions, in length as well 

 as laterally. This lateral growth would consequently necessitate a larger increase 

 of the mesoderm. Its strong development is not a matter of absolute necessity : a 

 good number of Calcarea (Ascones), though almost devoid of mesoderm, prosper not- 

 withstanding. The mesoderm is, however, a very important constituent part of the 

 organisation of the sponge, for in it the skeleton and generative elements are situated, and it 

 is evident that under certain circumstances its strong development might have proved to 

 be of great importance — the majority of sponges are rich in mesoderm, and we know 

 that its early development is one of the chief characters differentiating the Porifera from 

 the Ccelenterata proper. Its strong growth, however, according to the law of correlation 

 of the organs and with respect to Olynthus, cannot remain without consequence. Let 

 us now suppose — and this supposition w T ill be, if not proved in the scientific sense of 

 the word, at least shown to be probable — let us suppose that the cells which are charged 

 with the feeding of the sponge are chiefly pavement-cells, indifferently of ecto- or 

 endodermic origin, and not flagellated cells. The lateral growth in the Olynthus must 

 in any case have a limit, for, its walls becoming thicker, the outer surface covered with 

 pavement-cells must, sooner or later, according to geometrical laws, become too 

 small to feed the whole. If this limit be passed, a change in the organisation becomes 

 necessary, the surface covered with pavement-cells must, in some way or other, grow 

 larger. In order to form a Sycon, pocket-like invaginations are formed in the Olynthus ; 

 we have seen, however, that these invaginations, viz., radial tubes, bring about an 

 absolute not comparative enhu'ging of the outer as well as of the inner surface, and that, 

 alone, they would be of no use ; every thin-walled radial tube, like an independent Ascon, 

 w T ould be able to take care of its own feeding, but would be of no service to the compara- 

 tively thick-walled central tube representing the supporting apparatus of the whole. 

 We should therefore expect here a corresponding accommodation, and this accommodation 

 I see in the substitution of pavement-cells for the flagellated cells covering its inner 

 surface in the embryonal stage of development, the radial tubes taking upon themselves 

 alone the function of the circulation of the w r ater. To sum up, I formulate my conclu- 

 sions in the following manner : — 



The Ascones and the Sycones are two fundamentally different modifications of the 

 Olynthus, their chief distinction consisting in the unequal development of the mesoderm, 



